I think there are four interlocking reasons that make this a “mission impossible” from anyone but Apple:

Apple’s mobile device management model is very difficult to implement — even for leading corporate partners [1].

It’s non-trivial development to build something like scheduled app access control on top of Apple’s suite of iOS restrictions. This isn’t something schools and business need, so it has to be supported by the family market.

Very few people will pay for this service. It’s a lot of work for a niche market.

Any vendor looking at the home market knows that Apple could eliminate their business at any time with no warning. That’s what Google did with Family Link.

Only Apple can do the equivalent of Google’s Family Link [2]. That may require governmental pressure. Until Apple does it parents of children and guardians of special needs adults will need physical access to iPhones to implement restrictions.

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[1] JAMF is the dominant vendor in the corporate and educational iOS MDM market. I recently took advantage of a “Daring Fireball reader” special offer for a free 3 device JAMF account. When I enrolled a test device I discovered that annual certificate renewal disconnects enrolled devices (unless you have a dedicated corporate Apple ID) and I learned that full access to Apple’s suite of iOS restrictions requires either Apple’s “PreStage purchase program” or use of Apple Configurator (I think this is in flux with iOS 11 and 12).[2] If Apple does add MDM to iCloud, I hope they think about vulnerable adults. Google’s “age of consent” (13yo in US) opt-out and notification approach is a workable alternative to disabling use of Family MDM for adults.